1. Field of the Invention
The present invention relates to a color cathode ray tube and, more particularly, it relates to the structure of an envelope provided with plural necks and a color cathode ray tube provided with this envelope.
2. Description of the Related Art
The conventional color cathode ray tube provided with a plurality of necks is shown in FIG. 1. As disclosed in Japanese Patent Disclosure No. Sho 61-256551, this color cathode ray tube 1 includes envelope 11 comprising panel section 2 provided with a substantially rectangular face plate 4 and a skirt 6 extending from the circumferential rim of face plate 4, a funnel section 8 connected to a panel section 2, and a plurality neck sections 10 continuous from funnel section 8. The inside of cathode ray tube 1 is kept in a vacuum by the panel, the funnel and neck sections 2, 8 and 10. An electron gun assembly 12 for shooting three electron beams is housed in each of a neck sections 10. The funnel and neck sections 8 and 10 are provided, on the outer surface thereof, with a deflection means 14 for generating a magnetic field to deflect the electron beams in the horizontal and vertical directions. A phosphor screen 16 is formed on the inner surface of the face plate 4 at the panel section 2. A substantially rectangular shadow mask 18 is arranged inside the tube, facing the phosphor screen 16 with a certain interval interposed between them. The shadow mask 18 is made of a thin metal plate and is provided with a plurality of slit apertures 20. Frame 22 is attached to the circumferential rim of shadow mask 18.
Three electron beams shot from each of the electron guns 12 are deflected by deflection means 14. FIG. 1 shows those areas where the three electron beams are deflected. The electron beams thus deflected converge on the phosphor screen 16. The electron beams thus converged are introduced onto the phosphor screen 16 to emit the three colors of red, green and blue. Phosphor screen 16, which is scanned by the electron beams shot from electron guns 12, is sectioned to correspond to each of electron guns 12.
In the case of the envelope having the above-described structure to be used as the color cathode ray tube, the distance extending from the face plate to the neck can be made shortened as compared with another envelope, the same in size, but having only one neck section. This is because that area of the phosphor screen which is scanned by the electron gun housed in the neck section can be made smaller and the distance between the electron gun and the phosphor screen can thus be shortened. As the envelope is made larger in size, the effect of making this distance shorter can be enhanced further and further. In the case where the panel, funnel and neck sections are all made of glass, however, they cannot be made so sharply-curved as to make the distance between the electron gun and the phosphor screen shorter because the material of which they are made is glass. An envelope made of glass is therefore limited in how short the distance can be made.
The envelope having the above-described arrangement to be used as the color cathode ray tube has an extremely more complicated shape as compared with the other envelopes which are intended for the common color cathode ray tubes. In common color cathode ray tubes, this envelope is made of glass. It is therefore quite difficult to process glass to the complicated shape of this envelope. This makes it difficult to put this envelope into mass production.
This envelope is made to resist atmospheric pressure so as to maintain an inside vacuum. When it is large in size, quite a large force is added to it because of atmospheric pressure. It is therefore necessary for the thickness of the glass funnel section to be substantially the same as that of the glass panel section. However, a distortion of the glass neck section by the pressure is relatively small. It is therefore unnecessary to make the thickness of the glass neck section large. The glass neck section may be about 1 mm thick, for example. The thickness of glass thus changes sharply from the funnel to the neck section and this changes heat capacity sharply at the portion where the glass shifts from the funnel to the neck section. During manufacture, heat is applied to the envelope at a plurality processes thus increasing its thermal distortion. Because the envelope is made more likely to be broken because of thermal distortion, it is difficult to put the glass envelope on a mass production line. Further, the thickness of the funnel section is made substantially the same as that of the panel section. When the envelope is large in size, therefore, it becomes quite heavy. When it is heavy, it increases the weight of the color cathode ray tube for which it is intended.